Studies on factors regulating interphotoreceptor retinoid-binding protein (IRBP) gene expression in the mouse eye demonstrated that light deprivation markedly downregulated IRBP message in developing and adult mice. However, IRBP protein levels were not decreased, suggesting regulation by some sort of feedback control mechanism. Studies on retinoid levels in the eyes of the C57vit/vit mouse model of retinal degeneration showed markedly elevated levels of retinyl palmitate as well as some elevation of levels of 11-cis retinal, all-trans retinal an all-trans retinol at 6 weeks of postnatal development, suggesting a malfunction in some aspect of the visual cycle in this mutant. An effect of IRBP on the process of 11-cis retinal formation has been suggested by studies of retinoid processing in the toad retinal pigment epithelium (RPE) eye cup which show that both the molar amount and the specific radioactivity of 11-cis retinal increase with increasing IRBP concentration. Analysis of endogenously bound ligands of Drosophila head retinoid- binding protein demonstrated the presence of a possible isomer of retinol and both covalent and noncovalently bound fatty acids. We also characterized the binding affinities of retinol and fatty acid.